The present invention relates to a pattern measuring method for use with a scanning electron microscope which scans a specimen with electron beams and captures information on the surface of the specimen in the form of image from the amount of a secondary electron signal emitted from the specimen as a result of the scanning, and more particularly, to a method of stably measuring an average line width of a pattern.
With the increasing miniaturization of semiconductor devices, the edge roughness of a pattern has become a problem. The edge roughness has been investigated in regard to how it occurs, and attempts have been made for a management based on the roughness value, and a management based on an average measured value including the edge roughness used as an indicator.
FIG. 2 shows an example of a conventional line width measuring method. An edge detecting range is set for a pattern under measurement. The profile of an image is created from image information, and the positions of edges are determined using information on the waveform of a differentiated version of the profile. The line width is determined from information on the left and right edges (in the same orientation of adjacent patterns when in a pitch measurement). A portion corresponding to a convex edge of the pattern appears in white in a SEM image. Even a straight line by design results in an undulate form, as indicated by a black wave line in the figure. The edge roughness is defined to be variations in the noted right or left edge, while a width roughness is defined to be variations in the line width measured a plurality of times in the vertical direction.
The prior art related to the foregoing pattern measurement is disclosed, for example, in JP-A-2003-37139.
The width roughness will be described with reference to FIG. 3. The measurement of a line width involves setting edge detecting areas along the two edges, as indicated by black bold lines in FIG. 3, and detecting edges within the edge detecting areas. Then, the distance between the edges is defined to be a line width in that area. However, when roughness exists as shown, the line width largely differs at a measuring position A and at a measuring position B in FIG. 3.
Possible causes for the roughness can include variations attributable to the measuring method other than variations in the shape itself. FIGS. 4A and 4B show exemplary profiles at the measuring position A and measuring position B in FIG. 3, respectively. Generally, for making measurements at a plurality of measurement points, a common parameter is set and applied to each pattern under measurement. Assume that a line width is measured with thresholds which are 30, 50, 70% of the difference between the highest point and the lowest point in a profile. When the pattern does not include any local step as shown in FIG. 4B, measured values increase at a constant rate in the order 30%, 50% and 70%. In other words, there is a linear relationship between the threshold and the measured value. On the other hand, when a profile include local steps as shown in FIG. 4A, measured values irregularly vary depending on which of 30%, 50% and 70% threshold should be used. In other words, measured values largely vary depending on a set threshold. Since such steps can locally and sporadically appear in a pattern, measured values largely vary depending on an area used for the measurement and on a set threshold, even in a line pattern as shown in the example.
Consider that the foregoing measurement is applied to a line pattern which has local edge roughness as shown in FIG. 3. Portions represented by white bold lines are called “white bands” which correspond to the edges of the line, and include global pattern roughness. On the other hand, portions indicated by black thin lines represent the result of measurement. The latter is indicated by black thin line in order to distinguish the global pattern shape from the local roughness.
The pattern having the roughness as shown is measured at the measuring positions A and B. The luminance value is accumulated in a rectangular area (measuring area) around the measuring position A in the vertical direction to create its profile, and information on the edges at the measurement point A is acquired from the profile resulting from the accumulation. Information on the edges at the measuring position B is acquired in a similar manner. Consider now that the line width is measured from the distance between the respective edges at the measuring positions A, B. From a viewpoint of a stable measurement of the line width in the line pattern, it is desired to be able to measure the line width W, represented by the spacing between the white lines in FIG. 3, from which the influence of the roughness is omitted. However, the aforementioned method is adversely affected by the local roughness, so that a line width WA is measured at the measuring position A, while a line width WB is measured at the measuring position B, thus presenting largely differing results of measurement depending on the position set for the measurement. While the roughness is seemingly caused by a process, a material and the like, correct causes cannot have been so far identified, so that the global line width can be preferably measured separately from the roughness.
Also, conventionally, when an average line width is measured for a line or a space pattern image including edge roughness, the line width is locally measured at multiple positions, and measured values are averaged, thus implying problems of an extended processing time, instable calculated values, and the like.